Known instruments such as that described in British patent specification No. 1482 077 incorporate a transducer which when energised emits a pulse of ultrasonic waves, the waves propagating through a coupling medium to the surface of the structure under inspection. The transducer is set far enough from the surface to give a long reverberation time between successive echoes from the surface, echoes from defects within the structure being observed after the first surface echo and before the next reverberation. The coupling medium preferably absorbs little of the wave energy so as not to attenuate the pulse.
British Patent Specification No. 2 010 484 describes an ultrasonic scanner including five transducers rotatably supported, and energised in succession, so as to send an ultrasonic beam onto a fixed plane reflector and so to scan the beam across a sector. The scanned sector lies in a plane substantially perpendicular to the surface against which the probe is placed and is centred about an axis below but parallel to the surface. The scanner therefore provides a cross-sectional display rather than a plan position display of defects below the surface.